The present invention relates to a sewing machine for forming a seam of a desired length based on preset data.
There is known a sewing machine of the type described as disclosed in U.S. Pat. No. 3,209,713. According to the disclosed sewing machine, the operator determines the number of stitches to be formed on the basis of a desired seam length and a feed pitch, and presets the determined number of stitches. A seam of the desired length can be formed on the sewing machine simply by starting same. More specifically, the stitches formed after the sewing operation has started are counted, and the motor of the sewing machine is de-energized when the count reaches a preset count. For sewing a label to a work fabric, for instance, the operator calculates the number of stitches based on the size of the label and a given feed pitch and presets the calculated number of stitches, followed by a trial sewing operation. If the feed pitch setting is found to be wrong as a result of the trial sewing operation, or if better sewing is desired, the operator is required to reset the feed pitch. At this time, the operator is also required to set the number of stitches so as to form a seam of the predetermined length. As a consequence, the operator must carry out the complex procedure each time a trial sewing operation is effected.
Another conventional sewing machine capable of forming a seam of a desired length such as a buttonhole seam is disclosed in U.S. Pat. No. 4,343,249. The sewing machine has a presser frame for a presser foot for pressing a work fabric against the bed of the sewing machine, the presser frame being movable with the work fabric. The distance the presser frame is moved is detected by a potentiometer. Inasmuch as the sewing machine can be stopped by a detected signal from the potentiometer, a desired seam length can be fromed irrespective of variations in the feed pitch. However, the distance in which the presser frame is movable is limited, and a seam length longer than such distance cannot be formed.
U.S. Pat. No. 3,808,995 shows a sewing machine capable of detecting the distance a work fabric is moved. The disclosed sewing machine has a differential feed means for maintaining the surplus width of the work fabric, and a pole wheel for detecting the movement of the work fabric in order to feed the work fabric for a predetermined length over which the surplus width is to be maintained. Each time the pole wheel turns for an interval corresponding to the movement of the work fabric for 1 mm, one pulse is generated by an impluse preparation stage. Such pulses are counted, and a step motor is turned through one step each time the count reaches a preset count. When a control disk is rotated for one revolution by the step motor, the operation of the sewing machine is stopped. Since the sewing machine ceases to operate when the control disk makes one rotation, it can feed a work fabric of a predetermined length even if the feed pitch is varied. However, this sewing machine is complicated in construction and the operator's action is interfered with as the pole wheel is mounted on the sewing machine bed.